Of the 2 you listed, I would go 25-06 with 115 bergers or NBTs, IMO the 280 unless its an ackley doesn't have enough ***** to push the 160grn+ bullets that are typically used at longer ranges. I will also reccomend you take a look at the 6mm remington or 243AI, both are easy to load for and sending a 105 berger at 3150-3300fps is a better option in my opinion, I wouldn't hesitate to take a whitetail at 800 or a yote at 1000 with either of these 6mms.

Using H1000 or Retumbo or RE25 in the AI version you stand to gain 100-150fps over the standard 243. The big factor in the 243 is having a fast twist barrel, 1:8" or 1:8.5" twist and shooting the high bc bullets like the 105 berger hybrid with a g1 bc of .547 or the 115dtac with a g1 of .585, this is what gives you the extra reach in 6mm, more so than a bigger case or an ackley improved case. But a combination of the 243AI/105 berger hybrid at 3250fps from a 26" 8twist barrel will have no problem with yotes at a 1000yrds. It will still hit pretty hard.

Of the two calibers you mentioned, 25-06 & 280, I'd go with the 280. I own and like both calibers. Under 350 yards, the 25-06 puts deer down faster than anything I've ever seen. Past that distance it looses steam pretty fast. The 280, with it's heavier bullets with higher BCs, hold up to twice that distance. One suggestion I would make is that you get a barrel with the correct chamber and a fast enough twist to shoot the heavier Berger VLDs. Look at the ballistics tables on the two calibers. The choice is evident.

__________________ WHEN YOU HUNT LONG RANGE, SOME TIMES THE SHOT IS THE TROPHY.